Studies on the Method of Thermal Analysis of Organic Medicinals. V.<SUP>*3, *4</SUP> Semi-micro Apparatus for the Differential Thermal Analysis Permitting Direct Observation.

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A convenient DTA apparatus using two matched thermistors was designed and was easily and inexpensively constructed. Because of sensitivity of the thermistors for detecting temperature difference and of the advantage of the double cell consisting of a inner tube and a jacket, the sample size is much reduced even to a few milligrams. The arrangement of the cell is also effective for making the base line flatten. Another characteristic point of the apparatus is that it permits direct viewing of the sample during the course of measurement. Since it is often desirable to compare peaks in the DTA curve with visual changes, the arrangement is found very useful for the study of phase reactions of organic materials. Tests concerning the experimental conditions show that analysis can be performed with accuracy and reproducibility. Applications were done for the studies of thermal behaviors of ammonium nitrate, bromo-diethylacetylurea and α-bromoisovalerylurea. Direct transformation of ammonium nitrate from the modification of IV to II was occasionally noticed. It was also confirmed that there was clear correspondence between the fall and rise of the DTA curve of copper sulfate pentahydrate and the changes in color observed with eye. As a general application of DTA, a simple method for evaluating the heat of transition was proposed. It is based on direct comparison of two peaks by conducting DTA with a mixture of the sample and the inert substance of known heat of transition. The heat of polymorphic transition of potassium nitrate at 128∼129° was found to be 1.20kcal./mol. by using benzoic acid as an internal standard. It is thought that the method will have wide application, especially to organic materials. Also, discrimination of pyrabital form the corresponding mixture of aminopyrine and barbital was successfully done by DTA. The curve of the former shows two peaks which is due to the stable eutectic liquefaction and to melting, while that of the latter exhibits in addition to these peaks, one pair of endothermic and exothermic ones which is attributed to the metastable eutectic liquefaction and to the molecular compound formation, respectively. These results will suggest that DTA is generally applicable to the detection of molecular compound between organic medicinals.

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